Modified zinc ferrite oxidative dehydrogenation catalysts

ABSTRACT

Improved oxidative dehydrogenation catalysts are prepared by modifying a preformed zinc ferrite oxidative dehydrogenation catalyst with zinc oxide. The resulting catalyst compositions exhibit higher conversions and yields.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a process for dehydrogenating hydrocarbons.More particularly, this invention relates to the oxidativedehydrogenation of organic compounds in the presence of modified zincferrite catalyst compositions.

2. Description of the Prior Art.

Oxidative dehydrogenation processes wherein zinc ferrite catalystcompositions have been employed to convert saturated and/or unsaturatedhydrocarbons to more highly unsaturated hydrocarbons through removal ofhydrogen from such hydrocarbons are known in the art. See, for example,U.S. Pat. No. 3,303,235. However, such catalyst compositions do notretain their good initial activity and deteriorate rapidly under thereaction conditions of the oxidative dehydrogenation process.

Accordingly, it is an object of the present invention to providecatalyst compositions which, when employed in oxidative dehydrogenationprocesses, effect high conversions at high selectivities to the desiredproduct.

It is another object of the present invention to provide more stableand, hence, longer-lived catalyst compositions than heretofore employedin oxidative dehydrogenation processes.

SUMMARY OF THE INVENTION

In accordance with the present invention, a process is provided for theoxidative dehydrogenation of organic compounds which comprisescontacting an organic compound having from about 2 to about 20 carbonatoms and oxygen in the presence of a zinc ferrite catalyst compositionadditionally containing zinc oxide as a catalyst modifier, said zincoxide having been added to the zinc ferrite catalyst composition afterits formation from the zinc ferrite precursor mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the process of the instant invention, certain organiccompounds are dehydrogenated to less saturated compounds of the samecarbon number at elevated temperature in the presence of oxygen and thecatalysts of the instant invention.

The process of this invention may be applied to the dehydrogenation of awide variety of organic compounds. Such compounds normally will containfrom 2 to 20 carbon atoms, at least one ##EQU1## grouping, a boilingpoint below about 350°C., and such compounds may contain other elements,in addition to carbon and hydrogen such as oxygen, halogens, nitrogenand sulfur.

Among the types of organic compounds which may be dehydrogenated bymeans of the process of this invention are nitriles, amines, alkylhalides, ethers, esters, aldehydes, ketones, alcohols, acids, alkylaromatic compounds, alkyl heterocyclic compounds, cycloalkanes, alkanes,alkenes, and the like. Illustrative dehydrogenations which may becarried out by the process of this invention include propionitrile toacrylonitrile; propionaldehyde to acrolein; ethylchloride to vinylchloride; methyl isobutyrate to methyl methacrylate; 2 or 3chlorobutene-1 or 2,3-dichlorobutane to chloroprene; ethyl pyridine tovinyl pyridine; ethylbenzene to styrene; isopropylbenzene to α-methylstyrene; ethylcyclohexane to styrene; cyclohexane to benzene; ethane toethylene or acetylene; propane to propylene, methylacetylene, allene, orbenzene; isobutane to isobutylene; n-butane to butene and butadiene;n-butene to butadiene-1,3 and vinyl acetylene; methyl butene toisoprene; cyclopentane to cyclopentene and cyclopentadiene-1,3; n-octaneto ethyl benzene and orthoxylene; monomethylheptanes to xylenes; ethylacetate to vinyl acetate; methyl isobutyrate to methyl methacrylate;2,4,4-trimethylpentane to xylenes; and the like. Other materials whichare dehydrogenated by the process of this invention include ethyltoluene, alkyl chlorobenzenes, ethyl naphthalene, isobutyronitrile,propyl chloride, isobutyl chloride, ethyl fluoride, ethyl bromide,n-pentyl iodide, ethyl dichloride, 1,3-dichlorobutane,1,4-dichlorobutane, the chlorofluoroethanes, methyl pentane, methylethylketone, diethyl ketone, n-butyl alcohol, methyl propionate, and thelike.

The catalyst compositions of this invention are also useful for theformation of new carbon-to-carbon bonds by the removal of hydrogenatoms. For example, acyclic compounds having from 6 to about 16 carbonatoms and no quaternary carbon atoms are converted to cyclic compoundsof greater degree of unsaturation, e.g., n-hexene to benzene. Also,propene is converted to diallyl.

The preferred compounds which are dehydrogenated by the process of thisinvention are hydrocarbons having from about 3 to about 12 carbon atoms,including alkanes, alkenes, cycloalkanes, cycloalkenes, and aromaticcompounds having one or two alkyl side chains of from 2 to 3 carbonatoms. A preferred hydrocarbon feed for the process of this inventionwould be selected from the group of n-butane, n-butene, pentane orpentene including all isomers and mixtures thereof, the methyl butenes,the hexenes, ethyl benzene, etc., and mixtures thereof. Especiallypreferred are acyclic hydrocarbons having 4 to 5 contiguousnon-quaternary carbon atoms, such as butane, the butenes, the methylbutenes and mixtures thereof.

In the instant process, the organic compound is dehydrogenated in thepresence of oxygen. Oxygen may be fed to the reaction zone as pureoxygen, air, oxygen-enriched air, oxygen mixed with a diluent, and soforth. Oxygen in the desired amount may be added in the feed to thedehydrogenation zone and oxygen may also be added in increments to thedehydrogenation zone. The oxygen may be supplied in a manner such asdescribed in U.S. Pat. No. 3,420,911.

The amount of oxygen employed in the oxidative dehydrogenation processwill vary depending upon the particular compound being dehydrogenated,the number of hydrogen atoms being removed, and the conversion level.For example, in dehydrogenating butane to butene, less oxygen isgenerally employed than if the reaction were carried out to producebutadiene. Normally oxygen will be supplied in the dehydrogenation zonein an amount from about 0.2 to about 1.5, and preferably from about 0.3to about 1.2 mols of oxygen per mol of H₂ being liberated from theorganic compound. Expressed in terms of the organic compound beingdehydrogenated, the oxygen is supplied in an amount of from about 0.2 to2.0 mols per mol of organic compound to be dehydrogenated with apreferred range of from about 0.25 to 1.5 mols of oxygen per mol oforganic compound.

Preferably, the reaction mixture contains a quantity of steam or adiluent such as nitrogen. These gases serve to reduce the partialpressure of the organic compound; however, the functions of steam in thereaction are several fold in that the steam does not act merely as adiluent. Whenever steam is employed in the process of the instantinvention, it is employed in an amount generally of from about 2 toabout 40 mols of steam per mol of organic compound to be dehydrogenated,with an amount of from about 3 to about 35 mols of steam per mol oforganic compound to be hydrogenated being preferred. Especiallypreferred are amounts of from about 5 to about 30 mols of steam per molof organic compound to be dehydrogenated. Whenever a diluent is employedinstead of steam, such diluents generally may be used in the samequantities as specified for steam.

In one modification of this invention, halogen is present in thereaction gases. The presence of halogen in the dehydrogenation zone isparticularly effective whenever the compound to be dehydrogenated is asaturated hydrocarbon. Whenever halogen is employed in thedehydrogenation zone, it is provided as elemental halogen or a compoundof halogen which liberates halogen under the conditions of thedehydrogenation reaction. Suitable sources of halogen include hydrogeniodide, hydrogen bromide and hydrogen chloride; aliphatic halides suchas ethyl iodide, methyl bromide, methyl chloride, and 1,2-dibromoethane;cycloaliphatic halides; ammonium iodide, ammonium bromide, ammoniumchloride, sulfuryl chloride; metal halides including molten halides; andthe like. The halogen also may be liberated partially or entirely by asolid source as disclosed in the process of U.S. Pat. No. 3,130,241issued Apr. 21, 1964. Mixtures of various sources of halogen may beused. Whenever employed in the process of the instant invention, theamount of halogen employed (calculated as elemental halogen) is fromabout 0.0001 to about 1.0 mols of halogen per mol of the organiccompound to be dehydrogenated with an amount of from about 0.01 to about0.5 mols of halogen per mol of organic compound being preferred.

The catalyst compositions useful in the present invention include zincferrites containing, as the active components thereof, zinc, iron andoxygen in combination as hereinafter described and additionallycontaining free zinc oxide as a modifier, said modifier being added tothe catalyst composition subsequent to the formation of the zincferrite.

The zinc ferrite constituents of the instant catalyst compositionscomprise zinc ferrite of the empirical formula Zn_(x) Fe_(y) O_(z),wherein x will be from about 0.1 to 2, inclusive, and y can be in therange of about 0.3 to 12, inclusive, and z will vary depending upon thenumber of oxygen vacancies, but will usually be within the range ofabout 3 to 18, inclusive. Especially preferred are zinc ferritecompositions wherein the ratio of y to x is from about 2:1 to about 5:1.Although the modified zinc ferrite catalyst may be broadly defined ascontaining crystalline structures of iron, oxygen and zinc, certaintypes of catalysts are preferred. Zinc ferrite formation may beaccomplished by reacting an active compound of iron with an activecompound of zinc. By the term active compound is meant a compound whichis reactive under the conditions hereinafter decribed to form theferrite. The active compounds are suitably oxides or compounds which areconverted to oxides during the formation of the ferrite, such as organicand inorganic salts or hydroxides. Active compounds of iron and zincinclude the nitrates, hydroxides, hydrates, oxalates, carbonates,acetates, formates, halides, oxides, etc. For example, zinc carbonatemay be reacted with iron oxide hydrates to form zinc ferrite. Salts ofthe desired metals may be co-precipitated and the precipitate heated toform the ferrite. Desired ferrites may be obtained by conducting thereaction to form the ferrite at relatively low temperatures, that is, attemperatures lower than some of the very high temperatures used for theformation of some of the semi-conductor applications. Good results havebeen obtained by heating the ingredients to a temperature high enough toproduce the zinc ferrite, but at conditions no more severe thanequivalent to heating to 850°C. for 90 minutes in air. Generally, themaximum temperature will be less than 700°C. and preferably about 650°C.Methods for preparing zinc ferrite catalyst compositions suitable foruse in the process of this invention are disclosed in U.S. Pat. Nos.3,270,080; 3,284,536; 3,303,234-6; 3,303,238; 3,308,182; 3,334,152;3,420,912; 3,440,299; 3,342,890 and 3,450,787.

As is apparent from the empirical formula presented herein for zincferrite, the ratio of iron to zinc in such ferrite mixtures is notrestricted to the stoichiometric ratios as would be present in thesimple compound zinc ferrite. In the catalyst compositions of theinstant invention, there is present zinc ferrite compound as well as oneor more oxides of the constituent cations. For example, if the activecompounds are employed such that in the empirical formula y is about 3and x is 1, the catalyst composition formed therefrom will contain ironoxide in addition to the zinc ferrite formed. Similarly, the zincferrite precursor composition may comprise an excess of zinc over thestoichiometric amount to form the ferrite, in which case the resultingcatalyst will contain zinc oxide in addition to the zinc ferrite formed.Such zinc oxide is, however, to be distinguished from zinc oxide whichis added to the ferrite composition after its formation. Zinc oxidewhich is added to the zinc ferrite composition and not subjected to thecalcination temperatures employed during the ferrite formation processproduces an unexpectedly advantageous promoting effect.

The preferred zinc ferrite catalyst compositions of the instantinvention are those having a face centered cubic structure. However, thezinc ferrites of the instant invention will not be present in the mosthighly oriented crystalline structure because it has been found thatsuperior results may be obtained with catalysts wherein the zinc ferriteis relatively disordered. Such catalyst compositions may be obtained byconducting the reaction to form the zinc ferrite at relatively lowtemperatures as described herein.

The zinc ferrite catalyst compositions of the present invention can beidentified by their characteristic X-ray diffraction patterns. Thepreferred catalyst compositions will generally have X-ray diffractionpeaks at d-spacings within or about 4.83 to 4.89; 2.95 to 3.01; 2.51 to2.57; 2.40 to 2.46; 2.08 to 2.14; 1.69 to 1.75; 1.59 to 1.65; and 1.46to 1.52, with the most intense peak being between 2.51 to 2.57.Particularly preferred catalysts will have d-spacings within or about4.81 to 4.88; 2.96 to 3.00; 2.52 to 2.56; 2.41 to 2.45; 2.09 to 2.13;1.70 to 1.74; 1.60 to 1.64; and 1.47 to 1.51, with the most intense peakfalling within or about 2.52 to 2.56. These X-ray determinations aresuitably run with a cobalt tube.

The zinc oxide catalyst modifier of the instant invention can beemployed in the form of zinc oxide itself or a zinc compound which willbe converted to zinc oxide under the reaction conditions set forthherein. Particularly effective are inorganic zinc cmpounds such as theoxides and salts, including the phosphates, sulfates, phosphites,sulfites, silicates, thiocyanates, thiosulfates, and the like. Speciallypreferred are zinc oxide and zinc carbonate.

The zinc oxide or zinc oxide precursor catalyst modifier may be added tothe zinc ferrite by any suitable method. It is essential only that themodifier be added to the catalyst composition after such time as thezinc ferrite has been formed. If a catalyst support or carrier isemployed, one convenient method is to form a slurry of the modifier withthe zinc ferrite prior to coating on the support. Although aqueousmediums will generally be employed when coating a support with thecatalyst constituents, it is contemplated that non-aqueous systems canalso be employed, if desired, in the preparation of the catalyst.Another suitable method for incorporating the modifier into the zincferrite composition is by dry-mixing the components.

The zinc oxide modifier is present in the zinc ferrite catalystcomposition in a catalytic promoting amount. Generally, a catalyticpromoting amount of zinc oxide will be not more than about 25%, byweight, based on the total weight of the zinc ferrite compositionpresent. Amounts of zinc oxide of from about 0.1 to 25% aresatisfactory, with amounts of from about 1.0 to about 5.0%, based on theweight zinc ferrite composition being preferred.

Catalyst binding agents for fillers not mentioned herein may also beused, but these will not ordinarily exceed about 50 percent or 75percent by weight of the catalytic surface, and the described catalyticcompositions will preferably constitute the main active constituent.These other binding agents and fillers will preferably be essentiallyinert. Preferred catalysts are those that have as a catalytic surfaceexposed to the reaction gases at least 25 or preferably 50 weightpercent of the defined catalytic surface. The catalytic surface may beintroduced as such or it may be deposited on a carrier by methods knownin the art such as by preparing an aqueous solution or dispersion of acatalytic material and mixing the carrier with the solution ordispersion until the active ingredients are coated on the carrier. If acarrier is utilized, very useful carriers are silicon carbide, aluminumoxide, pumice, and the like. Other known catalyst carriers may beemployed. When carriers are used, the amount of catalyst on the carrierwill suitably be between about 5 to 75 weight percent of the totalweight of the active catalytic material plus carrier. Another method forintroducing the required surface is to utilize as a reactor a smalldiameter tube wherein the tube wall is catalytic or is coated withcatalytic material. Other methods may be utilized to introduce thecatalytic surface such as by the use of rods, wires, mesh, or shreds,and the like, of catalytic material. The catalytic surface described isthe surface which is exposed in the dehydrogenation zone to the reactiongases, that is, e.g., if a catalyst carrier is used, the compositiondescribed as a catalyst refers to the composition of the surface and notto the total composition of the surface coating plus carrier.

The catalyst compositions of the instant invention may be activatedprior to use by treatment with a reducing gas, such as, for example,hydrogen or hydrocarbons. For example, the reduction may be effectedwith hydrogen at a temperature of from about 500° to about 1,000°F.,with temperatures of from about 650° to about 850°F. being preferred.The time required for reduction will be dependent upon the temperatureselected for the reducing step and will generally be from about 10minutes to about 2 hours.

The catalyst compositions of this invention may also comprise additives,such as disclosed in U.S. Pat. No. 3,270,080 and U.S. Pat. No.3,303,238. Phosphorus, silicon, boron, sulfur, or mixtures thereof, areexamples of additives. Excellent catalysts may contain less than 5 wt.%,and preferably less than 2 wt.%, of sodium or potassium in the catalystcomposition. The catalyst compositions of this invention may alsocomprise other metallic promoters as are well-known in the art.

The Reaction Conditions

The temperature for the dehydrogenation reaction will depend upon thecompound being dehydrogenated and the desired level of conversion.Generally, temperatures of from about 500° to about 1,200°F. aresatisfactory with temperatures of from about 650° to about 1,100°F.being preferred.

The process of the instant invention is carried out at atmosphericpressure, superatmospheric pressure or at subatmospheric pressure. Thereaction pressure will normally be about or in excess of atmosphericpressure, although subatmospheric pressure may also desirably be used.Generally, the total pressure will be between about 2 p.s.i.a. and about125 p.s.i.a., with a total pressure of from 4 p.s.i.a. to about 75p.s.i.a. being preferred. Excellent results are obtained at aboutatmospheric pressure.

The gaseous reactants may be conducted through the dehydrogenation zoneat a fairly wide range of flow rates. The optimum flow rate will dependupon such variables as the temperature and pressure of reaction, and theparticular hydrocarbon being dehydrogenated. Desirable flow rates may beestablished by one skilled in the art. Generally, the flow rates will bewithin the range of about 0.10 to 15 liquid volumes of the organiccompound to be dehydrogenated per volume of dehydrogenation zonecontaining catalyst per hour (referred to as LHSV). Usually, the LHSVwill be between 0.15 and about 5.0.

In calculating space velocities, the volume of a fixed beddehydrogenation zone containing catalyst is that original void volume ofreactor space containing catalyst. The gaseous hourly space velocity(CHSV) is the volume of the hydrocarbon to be dehydrogenated, in theform of vapor calculated under standard conditions of 25°C. and 760 mm.of mercury, per volume of reactor space containing catalyst per hour.Generally, the GHSV will be between about 25 and 6400, and excellentresults are obtained between about 38 and 3800. Suitable contact timesare, for example, from about 0.001 or higher to about 5 or 10 seconds,with particularly good results being obtained between 0.01 and 3seconds. The contact time is the calculated dwell time of the reactionmixture in the reaction zone, assuming the mols of product mixture areequivalent to the mols of feed mixture. For the purpose of calculationof residence times, the reaction zone is the portion of the reactorcontaining catalyst.

The process of this invention is suitably deployed with a fixed catalystbed or a moving catalyst bed, such as a fluidized catalyst bed in thedehydrogenation zone.

The following examples are illustrative only of the invention and arenot intended to limit the invention. All percentages are weight percentunless specified otherwise. All conversions, selectivities and yieldsare expressed in mol percent of the designated feed.

EXAMPLE I

A. Preparation of a Zinc Ferrite Catalyst Composition

To approximately 35.3 liters of distilled water were added 8,603 g.ferric oxide, 3,733 g. zinc carbonate and 61.8 g. zinc chloride to forma slurry. The slurry was thoroughly mixed for 5 hours after which timeit was dewatered by filtering and the filter cake was dried in an ovenat 260°F. for 12 hours. The dried filter cake thus obtained wasgranulated and blended in a Patterson-Kelly blender with enough water toform moist granules. The granules were then dried at 260°F. for 12hours. After drying, the granules were calcined at 1,200°F. for 14minutes in the presence of oxygen to form a zinc ferrite-containingcatalyst composition. The zinc ferrite catalyst composition was analyzedby X-ray diffraction and found to contain zinc ferrite and approximately25 wt.% free or uncombined ferric oxide.

The dry zinc ferrite-containing powder was then placed in aPatterson-Kelly blender and mixed with an aqueous solution containing 2wt.% polyvinyl alcohol and 7 wt.% phosphoric acid to give a damp powderwith a moisture content of approximately 28 wt.%. The damp ferritepowder was then pelletized (1/16-inch pellets) in a pellet mill.

B. A total of 125 cc. of the pelleted catalyst composition producedaccording to the procedure of Part A of this example was used todehydrogenate butene-2 to butadiene-1,3 using a 25 mm. OD glass reactorapproximately 13 inches long in the heated reactor section. Butene-2 wasfed together with oxygen (as air) and steam over a fixed catalyst bed.The effluent gases from the reactor section were passed through a coldwater condenser to remove most of the steam and samples of the effluentgas were withdrawn with a syringe at the exit from the condenser andwere analyzed in a Perkin-Elmer vapor chromatograph. The butene-2 was CPgrade (99.0 mol percent minimum) and the oxygen was commercial gradepurity (99.5 mol percent).

Prior to use, the catalyst composition was pretreated by reduction for 3hours at 850°-1,050°F. in the presence of a fluent gas containing steamand hydrogen. Steam was employed at a GHSV of approximately 12-15 timesthe GHSV at which the butene-2 was to be passed over the catalyst duringthe oxidative dehydrogenation and the hydrogen flow rate through thereactor during the reduction step was 400 cc. per minute. After thereduction step, butene-2 was fed to the reactor at an LHSV of 1.5 alongwith air and steam. Data were recorded after the indicated hours ofon-stream operation listed in the following table. The data of Runs 2-4were obtained after the catalyst was subjected to a further reductionaccording to the above method in order to insure maximum catalystactivity.

                                      Table 1                                     __________________________________________________________________________    Run                                                                              Hours on                                                                            O.sub.2 /Steam/HC                                                                     Conversion                                                                           Selectivity                                                                          Yield                                                                             Maximum                                    No.                                                                              Stream                                                                              Mol Ratio                                                                             Mol %  Mol %  Mol %                                                                             Temp. °F.                           __________________________________________________________________________    1  19    0.55/12/1                                                                             42.8   92.3   39.5                                                                              1000                                       2  33.5  0.55/12/1                                                                             39.4   93.7   36.9                                                                               940                                       3  72.5  0.55/12/1                                                                             48.5   91.4   44.3                                                                              1000                                       4  205.5 0.55/12/1                                                                             48.6   88.7   43.1                                                                              1040                                       __________________________________________________________________________

EXAMPLE II

A zinc ferrite catalyst composition was prepared according to the methodof Example IA except that 1.5 wt.% zinc carbonate was added along withthe aqueous solution of polyvinyl alcohol and phosphoric acid and themixture was blended in a Patterson-Kelly blender. The resulting catalystcomposition was pelletized (1/16-inch pellets) and 125 cc. of thecatalyst was reduced and employed to dehydrogenate butene-2 according tothe method of Example IB. The data are listed in the following Table II.Runs 2 and 3 are data obtained after two subsequent reductions of thecatalyst composition.

                                      Table 2                                     __________________________________________________________________________    Run                                                                              Hours on                                                                            O.sub.2 /Steam/HC                                                                     Conversion                                                                           Selectivity                                                                          Yield                                                                             Maximum                                    No.                                                                              Stream                                                                              Mol Ratio                                                                             Mol %  Mol %  Mol %                                                                             Temp. °F.                           __________________________________________________________________________    1  225.5 0.55/20/1                                                                             69.8   94.7   66.1                                                                              870                                        2  572   0.65/12/1                                                                             71.9   93.6   67.3                                                                              960                                        3  592   0.75/12/1                                                                             74.4   91.0   67.7                                                                              1055                                       __________________________________________________________________________

The above data demonstrate that the incorporation of zinc oxide (in theform of a precursor, zinc carbonate) into the zinc ferrite forming stepprovides a vastly superior catalyst composition. With the zincoxide-modified catalyst, the conversion level of butene-2 was from 21.2to 35 mol percent higher at comparable selectivities and the yields werefrom 21.8 to 30.8 mol percent higher than for the unmodified zincferrite catalyst composition.

EXAMPLE III

For comparative purposes, a test was made employing a zinc ferritecatalyst composition containing zinc oxide which had been incorporatedinto the catalyst composition during the formation of the zinc ferrite.Such a catalyst composition is described in U.S. Pat. No. 3,303,235(Example 9). The catalyst was formed from iron nitrate and zinc nitrateemployed in amounts equivalent to 0.45 mol Fe₂ O₃ per 0.55 mol of ZnO.After decomposition of the nitrates, the zinc ferrite was formed bycalcining the mixture at 1,562°F. for 20 minutes. The resulting zincferrite catalyst composition contained zinc oxide. The catalyst wasemployed to dehydrogenate butene-2 at a flow rate of 1.0 LHSV, a steamratio of 30 mols and an oxygen ratio of 0.75 mols per mol of butene-2.At a reaction temperature of 842°F., the yield of butadiene-1,3 was 62mol percent. This represents a 4-5 mol percent lower yield than with thezinc oxide-modified zinc ferrite catalyst compositions preparedaccording to the procedure of Example II.

From the foregoing description and Examples of this invention, those ofordinary skill in the art may make many modifications and variationstherefrom without departing from the scope of the invention ashereinafter claimed.

We claim:
 1. A novel catalyst composition suitable for oxidativedehydrogenation of organic compounds produced by the process consistingessentially of mixing a powder of a preformed zinc ferrite compositionhaving the empirical formula

    Zn.sub.x Fe.sub.y O.sub.z

wherein x is from about 0.1 to about 2, y is from about 0.3 to about 12and z is from about 3 to about 18 wherein the ratio of y to x is fromabout 2:1 to 5:1 with powdered zinc carbonate as a promoter in an amountof from about 0.1 to about 25 wt. % determined on the basis of zincoxide and based on the weight of the preformed zinc ferrite compositionand aqueous phosphoric acid, said aqueous phosphoric acid being added inan amount to dampen the powder mixture of zinc ferrite and zinccarbonate.
 2. The composition of claim 1 wherein the zinc carbonatepromoter is present in an amount of from about 1.0 to about 5.0 wt. %determined as zinc oxide based on the weight of the preformed zincferrite composition.